The invention relates to a magnetic recording medium comprising a substrate of a nonmagnetic material bearing a magnetic thin layer having perpendicular anisotropy. The magnetic layer comprises alternate layers of Pd and Co.
The invention also relates to a method of manufacturing such a magnetic recording medium.
Thin magnetic layers with perpendicular anisotropy are used for the perpendicular method of magnetic recording of, for example, digital information. A considerably higher linear density (per unit of length of the information track) can be acquired than that which is provided by longitudinal magnetic recording. In perpendicular (sometimes termed vertical) magnetic recording the magnetic flux travels straight across the medium in alternate domains from one surface to the other instead of longitudinally parallel to the surface of the medium.
Information is stored (recorded) in the magnetic recording medium in the form of small areas (domains) in the magnetic layer with an opposite magnetization as compared with the surrounding areas. The information is read by a magnetic head which converts local discontinuities of the perpendicular magnetization and emanating fields into electrical signals. When the remanent magnetization in the thin layer is considerably smaller than the saturation magnetization, smaller subdomains of opposite magnetization or with a magnetization which is parallel to the thin layer instead of perpendicular thereto are present in the domains. The transitions between such subdomains cause an undesired electric signal (magnetic noise) when reading the magnetic information.
The strength of the information signal depends on the difference in remanent magnetization of two oppositely magnetized areas and hence is proportional to M.sub.R (.perp.). The strength of the noise depends on the difference between the saturation magnetization M.sub.S and the remanence M.sub.R (.perp.). From this it follows that a criterion for obtaining a large signal-to-noise ratio is that ##EQU1## must be large. In order to be able to obtain a strong signal, M.sub.S must also be large. Other known requirements which a magnetic recording medium with perpendicular anisotropy must satisfy are that the remanent magnetic ratio must be larger than 1, and the coercive force preferably is between 200 and 3000 Oe.
In the article entitled "Perpendicular magnetic anisotropy in Pd/Co thin film layer structures" by P. F. Carcia et al. (Applied Physics Letters, Volume 47 No. 2, pages 178-180 (July 15, 1985)), the use of sputtered alternate layers of nonmagnetic Pd and magnetic Co is described for perpendicular magnetic information storage. The thicknesses of the Co-layers are 0.47 to 0.72 nm and the thicknesses of the Pd-layers are 1.06 to 7.84 nm. The saturation magnetization 4.pi. M.sub.S is 1480 to 6010 G, the coercive force H.sub.C is 350 to 550 Oe, and the remanent magnetic ratio (which is the ratio of the remanence measured perpendicular to the thin layer to the remanence measured parallel to the layer, M.sub.R (.perp.)/M.sub.R (.parallel.)) is 1.1 to 10.
The value of the remanent magnetic ratio indicates the easy axis of magnetization and in a thin film with perpendicular anisotropy it exceeds 1. The thin layers with the greatest saturation magnetization prove to show the smallest remanent magnetic ratio. Moreover, comparative experiments have demonstrated that such thin layer structures only show a small magnetic remanence M.sub.R when the saturation magnetization M.sub.S is large.